The present invention relates generally to octagonal phase rotators.
A phase rotator is a device which produces an output signal, wherein the phase is dependent on a digital control code supplied to the device. Phase rotators are used, for example, in I/O (Input/Output) link transceiver circuits of data processors and communications systems to rotate the phase of an input reference clock signal to obtain an output clock signal with a desired phase for data transmission/recovery. The phase rotator apparatus typically comprises a set of DACs (digital-to-analog converters) which steer current between different phases of an I/Q (in-phase and quadrature) reference signal in dependence on particular bits of the digital control code, and a set of amplifiers which receive and weight the phases of the I/Q reference signal, based on current steered to each phase by the DACs. An output circuit sums the weighted output signals from the amplifiers to produce the phase-adjusted output signal. In effect, the phase rotator operates as a current-steered mixer which generates the output signal by interpolating between the different phases of the reference signal in dependence on the input control code.
Octagonal phase rotators typically comprise three DACs which may be referred to as an I-DAC, a Q-DAC and an IQ-DAC. The I-DAC steers current between opposite polarities (positive and negative phases, “IP” and “IN” respectively) of the in-phase (I) signal. The Q-DAC steers current between positive and negative phases (“QP” and “QN” respectively) of the quadrature (Q) signal. The IQ-DAC steers current between all phases IP, IN, QP, QN. In particular, the IQ-DAC steers current between the I and Q signals, and also switches the current between the positive and negative phases of each signal. This switching is effected by I and Q polarity switches in the IQ-DAC. The rotator thus interpolates between the four phases IP, IN, QP, QN. The digital control code controls operation of the DACs such that a 360-degree phase rotation in the output signal, produced by successive incremental changes in the code, produces an octagonal phase envelope on an I/Q phase diagram representing how current is steered between the four phases. The polarity switches in the IQ-DAC are activated at quadrant crossings of the I/Q phase envelope, i.e. when the phase changes between IP and IN, or between QP and QN. An example of such an octagonal phase rotator is described in “A 16 Gb/s 3.7 mW/Gb/s 8-Tap DFE Receiver and Baud-Rate CDR With 31 kppm Tracking Bandwidth”, P. A. Francese et al., IEEE Journal of Solid-State Circuits, Vol. 49, No. 11, 2014, pp. 2490-2502.